Method for continuously producing nitrohumic acids



Oct. 20, 1964 KOZO HIGUCHI ETAL 3,

METHOD FOR CONTINUOUSLY PRODUCING NITROHUMIC ACIDS Filed Aug. 3, 1959 4 Sheets-Sheet 1 Fig.

Amount of N0 gas Generated gas/kg coal/min.)

Reaction Time (mm) INVENTOR S ATTORNEY Oct. 20, 1964 KOZO HIGUCHI ETAL 3,153,666

METHOD FOR CONTINUOUSLY PRODUCING NITROHUMIC ACIDS Filed Aug. 3, 1959 4 Sheets-Sheet 2 I lg. 2

Amount of Heat Evolved.

( K Cal/ kg coal/min.)

' Reaction Time (mi n.)

INVENTORS KOZO HIGUCHI MIGHIO TSUIUGUCHI Oct. 20, 1964 KOZO HlGUCHl ETAL 3,153,666

METHOD FOR CONTINUOUSLY PRODUCING NITROHUMIC ACIDS Filed Aug. 3, 1959 4 Sheets-Sheet 5 Fig.3

Yield of Nirrohumic Acid by weight) 60 I20 I80 240 3G0 360 Reaction Time (min) INVENTOR 3 X020 HIGUCHI MICHIO TSUEUGUGHI ATTORNEY Oct. 20, 1964 KOZO HIGUCHI ETAL 3,

METHOD FOR CONTINUOUSLY PRODUCING NITROHUMIC ACIDS Filed Aug. 3. 1959 4 Sheets-Sheet 4 INVENTOR 8 R020 HIGUCHI MICHIQ TSUYUGIIOH! BY 1/ v ATTdRNEY United States Fatent Gfice dd aihbi Patented Get. 20, 1964 This invention relates to a method and an apparatus for the continuous production of nitrohumic acids. More particularly, it relates to a novel method of oxidizing continuously with dilute nitric acid low rank coals such as brown coal, lignite and the like and to a novel continuous oxidizing apparatus adapted for practicing this method.

An object of this invention is to provide a method of producing nitrohumic acids on a large scale at low cost and an apparatus suitable for practicing same.

Another object of the invention is to provide a continuous oxidizing method for low rank coals that makes possible a notable increase in the rate of recovery of nitric acid which occupies a greater part of the direct cost in the production of nitrohumic acids and also an apparatus suitable for practicing said method.

Other objects and advantages of this invention will become apparent from the explanation given hereinafter.

Heretofore, in the production of nitrohurnic acids from such low rank coals as brown coal and lignite, the method employed has been that in which a reaction vessel is charged with said coals and dilute nitric acid whose concentration is about 5-40%, reacted for about 2-5 hours at a temperature below the boiling point of the added dilute nitric acid while being mixed and stirred, then obtaining by filtering out the insoluble solid part containing large quantities of nitrohumic acids, and thereafter drying same. Hence, as is evident from the foregoing, a non-continuous batch type method using a single reaction vessel was the method that was heretofore employed.

When We engaged in an intensive study of the process of producing nitrohumic acids by reacting dilute nitric acid with coals of low rank, what we found in this reaction was that an initial-stage reaction occurred in which was generated a conspicuous quantity of reaction-formed gases containing large quantities of nitric oxide (N) gas which was accompanied simultaneously with a notable amount of heat of reaction; and that this initial-stage reaction ended after about 5-30 minutes and was followed by a latter-stage reaction necessary for substantially forming the nitrohurnic acids, which lasted for one to four hours. And in this latter-stage reaction, it was found that the reaction-formed gases as well as heat produced was very much less than in case ofthe initial-stage reaction. While referring to the attached drawings, FIGURES l, 2, and 3, the above facts will be explained.

FIGURE 1 shows the relationship between the reaction time and the amount of nitric oxide (N0) gas produced when coal produced by the Teshio coal field of Hokkaido, Japan, having an elementary composition as shown in Table I, below, was used as the raw material and a 20% by weight nitric acid was added in an amount of live times by weight of the raw material and reacted by the batch method at 90 C., the relationship being shown by expressing the amount of nitric oxide (N0) gas produced in liter of gas/kg. of coal/min.

TABLEI Percent C 77.36 H 6.57 N 0.98 S 0.10 0 (difference) 14.99 Ash 5.14

Note.'lhe elementary analysis values in Table I have been expressed on a dry ash-free basis.

FIGURE 2 shows the relationship between heat of reaction and reaction time when an experiment in which the reaction conditions identical to that of FIGURE 1 was carried out, the relationship being shown by expressing the generating condition of heat of reaction in koaL/ kg. of coal/min.

And FIGURE 3 is that which shows the yield in percentages by weight of the nitrohumic acids with respect to the lapse of time when the reaction was carried out under identical conditions as in the experiment of FIGURE 1 in a similar manner as in the case shown in FIGURE 2.

As is clear from the attached drawings, FIGURE 1, 2, and 3, in the method of producing nitrohumic acids by the batch method, it can be seen how very difficult it is to recover effectively nitric acid from the nitric oxide (N0) gas generated during the reaction. This is because, if nitric acid recovery system is designed so as to make possible the handling of the excessive amount of gas generation that occurs during the initial stage, the nitric acid recovery system would not only have to be very extensive, but also during the latter-stage reaction when the gas generated becomes very small, the recovery of nitric acid would become unsatisfactory. In fact, according to the researches made by us, it was found that the recovery of nitric acid in the batch method never exceeded 30% of the theoretical yield.

Furthermore, when considered from the viewpoint of reaction temperature control in the reactive production of nitrohumic acids by the batch method, as is evident from FIGURE 2, since very great amount of heat is evolved in the initial-stage reaction, it is necessary to provide for forced cooling ofthe reaction system for maintaining the reaction system at a constant temperature to make possible the uniform carrying out of the reaction. On the other hand, since the heat produced during the latter-stage reaction is so small that with this alone it is not possible to maintain the temperature of the reaction system at the same level as that during the initial-stage reaction, the necessity arises of heating the reaction system instead. Moreover, in order to initiate the reaction for producing nitrohumic acids, it is first necessary to raise the temperature of the reaction system to that required for initiating the reaction. Inasmuch as the repetition of the steps of heating and cooling in a single reaction vessel is not only very troublesome from the operational standpoint, but also because the maintenance of a uniform reaction temperature is diflicult, it is almost form quality each time.

It was also observed from researches made by us that the reaction of coal and dilute nitric acid makes substantially no progress at below 30 C., and although differences existed depending upon the kinds of raw material coals used that generally for the reaction for producing nitrohumic acids at least 40 C., preferably a temperature of about 50l00 C., was suitable.

By the method of this invention which comprises passing in succession the suspension of powdered low rank coals and dilute nitric acid with a concentration of about 5-4070 by weight through at least three reaction vessels in series, gathering collectively on the other hand from each of said reaction vessels the reaction-formed gases produced and channelling same to the nitric acid recovery system, while maintaining the whole reaction system under normal pressure and within the temperature range of 40l00 C., it is possible to eliminate almost completely the defects of the batch process as described hereinabove.

The nitric oxide (N0) gas formed by the reaction of coal and dilute nitric acid, being oxidized by air or oxidized by air or oxygen, becomes nitrogen peroxide (N0 reacts with water in the nitric acid recovery system, and is recovered as nitric acid. in these processes, it it is possible to make constant the concentration of the nitric oxide (N0) gas and the amount of the reactionproduced gases per unit time, it becomes possible to make constant the flux of such as the added air or oxygen, or further the water, and consequently, the recovery of nitric acid can be effectively performed.

According to this invention, the gases produced by each of the plurality of reaction vessels connected in series are gathered collectively into a single pipe and then channelled to the nitric acid recovery system. In this case, each of the reaction vessels is by itself maintained at a constant reaction condition as far as the time factor is concerned producing regularly a constant quantity of reaction-formed gases containing nitric oxide (N0) gas of constant concentration. Now, even if the quantity of the reaction-formed gases and the concentration of the nitric oxide (N0) gas contained therein differ with respect to each of the reaction vessels, inasmuch as the gases produced by these reaction vessels are finally captured collectively and channelled to the nitric acid recovery system, the quantity of the gas to be treated by the nitric acid recovery system and the concentration of the nitric oxide (N0) gas contained therein are maintained at constant values at all times.

Moreover, since according to this invention each reaction vessel is maintained at constant reaction conditions, the heat input and output with respect to each reaction tank becomes constant, and thus the quantiy of heat to be either supplied, or to be removed in order to accomplish cooling, becomes constant as far as the time factor is concerned for each of the individual vessels. Hence, it is not necessary in carrying out the reaction to perform the complex operations of heating and cooling by the lapse of time as in the instance of a single vessel such as in the batch process. As a result, in accordance with this invention instead of heating and cooling a single vessel, it is possible to carry out each of the steps of the reactions necessary in producing nitrohumic acids in the respective vessels under steady states considering each vessel as a unit. And, as already described, in the reaction for producing nitrohumic acids, to begin with, there is the first step in which the reaction system is heated to a temperature necessary for starting the initial-stage reaction, the second step in which the initial-stage reaction is carried out wherein the reaction system must be cooled to maintain it at a given temperature because of the enormous amount of heat produced as a result of the reaction, and the third step in which the latter-stage reaction is carried out wherein the reaction system must be heated because a conspicuous amount of heat does not accompany the reaction despite the fact that this is the step in which substantially nitrohurnic acids are produced. According to this invention, the suspension of coal and dilute nitric acid is made to pass through at least three reaction vessels. The reason for this will be explained as follows: When three reaction vessels are employed, in the first vessel it is only necessary to perform constantly the above-described first step of heating for initiating the reaction; in the second vessel, constant cooling necessary for removing the excessive heat evolved in the initial stage of the reaction need only be performed; while the third vessel likewise will require only heating sufilcient to maintain the temperature necessary for the latter-stage reaction in a constant rate. However, in order to improve the rate of reaction of this invention and to raise the yield of nitrohumic acids, it is preferable that the number of reaction vessels be increased to more than five. Such a case, for example where five vessels have been employed, will be described. In this instance, the first reaction vessel can be heated for use in the first step; the next one or two reaction vessels can be cooled for use in the second step; and the final three or two can be heated for use in the third step. When more than six vessels are employed, the distribution of the vessels to the three groups can be accomplished using the above as a guide. For the purpose of convenience the locus of the above-described first step of heating for initiating the reaction may be called the first reaction Zone; the locus of the above-described second step of cooling may be called the second reaction zone; and the locus of the above-mentioned third step of heating, the third reaction zone.

For ideally accomplishing this invention, normally the number of reaction vessels of less than 20 is sulficient. In general, it is not advisable to increase the number of vessels to more than this number, since a conspicuous improvement in the rate of reaction attained cannot be expected as compared with the increase in construction costs involved and the need for greater expenses in the operation of the equipment.

With the specific gravity of the powdered coal much greater than that of dilute nitric acid in the suspension of powdered coal and dilute nitric acid for producing nitrohurnic acids, there exists the tendency to sedimentation of the powdered coal. Hence, it is most suitable to provide for maintaining a uniform state of suspension in each of the reaction vessels by providing agitators therein and to perform the transfer of the suspension between the vessels by means of an overflow system. Because, if an overflow system is not employed, not only would the sedimentable powdered coal clog the connecting pipes between the vessels or have tendencies to remain long in given spots, but there would also be the matter of greater complexity in the design, operation, etc. of the equipment.

Next, while referring to FIGURE 4, an embodiment suitable for practicing the above method of this invention will be described. However, the following embodiment being merely described for purpose of illustration, this invention should not be limited thereby, but only in so far as the same may be limited by the appended claims.

in FIGURE 4, 1 is the mixing vessel; 2, the dilute nitric acid supply; 3, the coal supply; l, the supply pipe; and d, the constant supply delivery pump. The reaction vessels 7, Nos. 1 to 9, disposed successively on top of an inclining rack 6, being each provided with an agitator 8 and a cooler 9, are connected in series. And the coolers 9 are connected in parallel to the main pipe 11 by means of the exhaust tubes 1%. The reaction product flowing out of No. 9 reaction vessel '2 is collected by the storage tank 12, and the main pipe 11 is connected at its one end to the nitric acid recovery apparatus 13.

in the reaction apparatus illustrated, the coal which has beenground is mixed with dilute nitric acid and other materials in the vessel 1 under given conditions, and after contact of its surfaces have been thoroughly accomplished with the reaction liquidand .it has been placed in condition where reaction can start immediately, while preventing its sedimentation by suitable agitation, it is TABLE II Experiment No 1 2 3 4 Rate of Rate of Rate of Rateot Vessel Reaction Vessel Reaction Vessel Reaction Vessel Reaction No. Attained No. Attained No. Attained No. Attained (Percent) (Percent) (Percent) (Percent) Reaction Time (min):

2 51. 9 1 46. 4 1 46. 4 1 46. 4 3 61.1 4 68. 5 2 63. 6 5 74. 6 2 68. 7, 6 79.5 3 75.3 7 83. 4 2 72.5 8 86.5 4 83. 3 3 81. 7 9 89.1 10 91. 2 5 88.6 3 85. 9 ll 92. 9 4 89. 3 12 94.3 6 92. 3 13 95.4 14 96. 3 7 94. 8 5 93. 7 4 92. 7 15 97.0 16 97. 7 8 96. 5 17 98. 0 6 96. 3 18 98. 4 9 97. (i 5 96. 3

In Table II, above, Experiment No. 1 is that in which idelivered into thebottcm of the reaction vessel 7 in uniform quantities by means of the constant supply deliver pump 5. As to the Nos. 1 to 4 reaction vessels 7, in these cases the vessels are provided with space twice that is actually used and thus makes possible the control of the violent reaction. The reacting materials enter each re- :action vessel at the bottom and leave the top to enter zthe next reaction Vessel by overflow means. Since it was confirmed that the fluidity of reacting materials was almost in an ideally mixed state, it being possible to sub- :stantially eliminate those which are discharged without the lapse of the required average residence time by in- 'creasing the apparent residence time by about 10-20% and the proper selection of the number of reaction vessels, the problem of the lack of uniformity in the residence 1 time of the coal particles which was the point of greatest rdifiiculty in the continuous process could be solved.

Thus, the gas which is produced when the coal par- "ticles are being oxidized while passing successively through each of the reaction vessels, after passing through ithe coolers and gas collecting tubes of a given sectional :area appurtenant to each of the reaction vessels are collected in the gas collecting main pipe and delivered to the nitric acid recovery apparatus. Hence, the gas to be absorbed that enters the recovery system not only possesses at all times an identical composition but also its volume shows hardly any change at all.

Therefore, the nitric acid recovery operation can be performed effectively, and at least 95% of nitric oxide (NO) contained in the reaction-formed gases can be recovered as nitric acid.

Next, the method of this invention will be described by means of an example, it being understood that the same also is merely intended in an illustrative sense, and the invention should not be limited thereby, but, only in so :far as the same may be limited by the appended claims.

Example The apparatus as shown in FIGURE 4 was employed, the number of reaction vessels used being 18, 9, 6, and 5 in Experiments Nos. 1, 2, 3, and 4, respectively, as shown in Table H, below.

Coal and nitric acid identical to those used in the experiment Whose results are shown in FIGURE 1 were used and nitro'numic acids were produced continuously .by means of the apparatus shown in FIGURE 4 with,

an apparatus consisting of 18 reaction vessels connected continuously in series in a manner similar to that of FIG- URE 4 was used. And likewise Experiments Nos. 2, 3, and 4 were those in which apparatuses consisting of nine, six, and five reactions vessels connected in series similarly as shown in the apparatus of FEGURE 4 was used.

In obtaining the rate of reaction attained of Table II, above, the value by the batch process at 240 minutes was considered as 109% and with this as the basis the rate of reaction attained was obtained by measuring the quantity of nitric acid contained in the solution discharged from the final reaction vessel.

As can be seen from Table 11, above, when 18 vessels were used and the total reaction time was 270 minutes, the rate of reaction attained was 98.4%. In Experiment No. 2 in which nine vessels were used and the total reaction time was also 270 minutes, the rate of reaction attained was 97.6%. Further, when six vessels were used and the reaction time was 255 minutes as in Experiment No. 3 and also five vessels were used and the reaction time was 270 minutes as in Experiment No. 4, the rates of reaction attained were 96.3% in both instances.

From the foregoing results, it can be seen that according to this invention by selection of the number of reaction vessels appropriately as to be adaptable to the reaction conditions such as the properties of the material coal, concentration of the nitric acid, etc., it is possible to attain the rate of reaction of at least 97% as against the rate of reaction attained by the batch method and thus the results compare favorably with the latter, manifesting hardly any inferiority. Moreover, as already mentioned hereinabove, it canbe seen that the operations such as temperature control of each of the reaction vessels and the recovery of the nitric oxide (N0) gas as nitric acid that are performed during the oxidizing reaction for obtaining these results can be carried out with the greatest of eficiency and smoothness, and moreover with ease. Furthermore, as is evident from the results shown in Table II, in case the reaction time is the same, the greater the number of reaction vessels, the rate of reaction attained shows improvement. However, as the number of reaction vessels increase, since the rate of improvement in the rate of reaction attained resulting from the increase in the number of reaction vessels decreases, it can be seen that a conspicuous improvement in' the rate of reaction attained eneaeee cannot be achieved proportionately with any excessive increase in the number of reaction vessels. This tendency is particularly evident when the number of vessels is in creased beyond twenty.

What is claimed is:

A method for producing nitrohumic acids which comprises continuously passing a suspension of powdered low rank coal in an aqueous nitric acid solution having a concentration of from about to about 40% by Weight acid into a first reaction zone, wherein said suspension is heated to a temperature above about 40 C., but below about 100 C., whereby an exothermic reaction between said coal and said nitric acid is initiated as evidenced by an initial evolution of nitric oxide gas from said suspension, upon said initiation of said reaction between said coal and said nitric acid withdrawing said heated suspension from said first reaction zone at a temperature below about 100 C., passing said withdrawn suspension from said first reaction zone to a second reaction zone wherein said suspension is maintained for a time period of about 5 to 30 minutes While being cooled to maintain the temperature thereof in the range of from about 40 to about 100 C,, withdrawing said cooled suspension from said second reaction zone at a temperature above about 40 C.,

passing said withdrawn suspension from said second reaction zone to a third reaction zone wherein said suspension is maintained for a time period of about 1 to 4 hours while being heated to maintain the temperature thereof in the range of from about 40 to about C. to provide therein a suspension, the insoluble portion of which contains substantial quantities of nitrohumic acids, withdrawing from said third reaction zone said nitrohumic acid-containing suspension, continuously collectively Withdrawing a gas stream containing nitric oxide overhead from said first, scond and third reaction zones and delivering said collective nitric oxide stream to a nitric acid recovery system, said first second and third reaction zones being maintained at atmospheric pressures.

References Cited in the file of this patent UNITED STATES PATENTS 442,298 Wiselogel Dec. 9, 1890 584,908 Venuleth June 22, 1897 804,516 Askenasy et al Nov. 14, 1905 1,432,761 Koch Oct. 24, 1922 2,015,044 Teichmann et al Sept. 17, 1935 2,461,740 Kiebler Feb. 15, 1949 2,555,410 Howard June 5, 1951 

